Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another to form the cylinders. To control the charge exchange, an internal combustion engine requires control elements—generally in the form of lifting valves—and actuating devices for actuating these control elements. Each lifting valve moves, so as to realize, that is to say perform, a valve lift, between an open position and a closed position, and in so doing, during an opening duration, opens up the opening associated with the valve. The valve actuating mechanism required for the movement of a valve, including the valve itself, is referred to as the valve drive. The cylinder head often serves to accommodate the valve drive.
During the charge exchange, the combustion gases are discharged via the outlet openings of the cylinders and the charging of the combustion chambers with charge air takes place via the inlet openings. It is the object of the valve drive to open and close the inlet and outlet openings at the correct times, with a fast opening of the largest possible flow cross sections generally being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure the best possible charging of the cylinders and an effective discharge of the exhaust gases. According to the prior art, therefore, the cylinders are also often provided with two or more inlet and outlet openings.
According to the prior art, the inlet ducts which lead to the inlet openings, and the exhaust lines which adjoin the outlet openings, are at least partially integrated in the cylinder head. The exhaust lines of the cylinders are generally merged to form one common overall exhaust line, or else in groups to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to generally as an exhaust manifold.
Downstream of the manifolds, the exhaust gases may be supplied, for the purpose of supercharging the internal combustion engine, to the turbines of at least two exhaust-gas turbochargers, and if appropriate to one or more exhaust-gas aftertreatment systems.
The advantage of an exhaust-gas turbocharger for example in relation to a mechanical charger is that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
An exhaust-gas turbocharger comprises a compressor and a turbine which are arranged on the same shaft. The hot exhaust-gas flow is fed to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the at least two cylinders is obtained. A charge-air cooling arrangement may be provided, by means of which the compressed charge air is cooled before it enters the cylinders.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased. Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. If the swept volume is reduced, it is thus possible, given the same vehicle boundary conditions, to shift the load collective toward higher loads, at which the specific fuel consumption is lower.
It is basically sought to arrange the turbine of an exhaust-gas turbocharger as close as possible to the outlet openings of the cylinders in order thereby to be able to make optimum use of the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and to ensure a fast response behavior of the turbine and thus of the turbocharger. In this connection, it is also sought to minimize the thermal inertia and the volume of the line system between the outlet openings of the cylinders and of the turbine, which may be achieved by reducing the mass and the length of the exhaust lines. Here, the integration of the exhaust manifolds into the cylinder head is expedient in achieving this aim.
The configuration of the exhaust-gas turbocharging often poses difficulties, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges. According to the prior art, however, a torque drop is observed in the event of a certain engine speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. This has the result that, toward lower engine speeds, the charge pressure ratio and the charge pressure likewise decrease, which equates to a torque drop.
According to the prior art, it has been sought to improve the torque characteristic of a supercharged internal combustion engine by various measures, for example by means of a small design of the turbine cross section and simultaneous exhaust-gas blow-off. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas flow rate exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. Said approach has the disadvantage that the supercharging behavior is inadequate at relatively high engine speeds or in the case of relatively large exhaust-gas flow rates.
The torque characteristic may also be advantageously influenced by means of multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower engine speed range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine, wherein a shut-off element is arranged in the bypass line in order to control the exhaust-gas flow conducted past the high-pressure turbine. The response behavior of an internal combustion engine supercharged in this way is considerably improved in relation to a similar internal combustion engine with single-stage supercharging, because the relatively small high-pressure stage is less inert, that is to say the rotor of a smaller-dimensioned exhaust-gas turbocharger can be accelerated more rapidly.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein individual turbines are activated successively with increasing exhaust-gas flow rate. At least one turbine may be in the form of an activatable turbine which is acted on with exhaust gas, that is to say activated, only in the presence of relatively high exhaust-gas flow rates.
To further improve the torque characteristic, each cylinder of the internal combustion engine may be equipped with at least one activatable outlet opening. The exhaust lines of at least two cylinders are then merged in grouped fashion such that the exhaust lines of the activatable outlet openings and the exhaust lines of the other outlet openings are merged, in each case with the formation of an exhaust manifold, to form an overall exhaust line.
The exhaust lines of the activatable outlet openings lead to the turbine of the first exhaust-gas turbocharger, and the exhaust lines of the other outlet openings lead to the turbine of the second exhaust-gas turbocharger. The first turbine, which is assigned to the activatable outlet openings, is thus in the form of an activatable turbine. According to the prior art, the activatable outlet openings are opened during the course of the charge exchange, and the activated first turbine is thus acted on with exhaust gas, only in the presence of relatively large exhaust-gas flow rates.
In comparison with concepts in which a single coherent exhaust line system is provided upstream of the two turbines, the above-described grouping, that is to say the use of two mutually separate exhaust manifolds, noticeably improves the operating behavior of the internal combustion engine, in particular at low exhaust-gas flow rates, inter alia because the line volume upstream of the second turbine, through which exhaust gas flows continuously, is reduced in size by this measure, which is advantageous, and in particular improves the response behavior, at low loads and engine speeds, that is to say in the presence of low exhaust-gas flow rates.
There are however also resulting disadvantages. The rotational speed of the activatable turbine decreases drastically when the latter is deactivated, such that, upon reactivation, the rotor of said turbine must initially be accelerated in order to be able to generate and provide the desired charge pressure at the compressor side. The response behavior is consequently impaired.
To ensure a minimum rotational speed of the activatable turbine, the activatable turbine could be supplied with a small exhaust-gas flow even when its outlet openings are deactivated. For this purpose, it would be necessary for a corresponding line to connect the second exhaust manifold to the first turbine, possibly with the use of at least one additional shut-off element, though this would disadvantageously increase the complexity and space requirement of the exhaust line system upstream of the turbines. Furthermore, the line would create a connection between the two exhaust manifolds, and the grouping described above would be eliminated. The effects obtained through the use of two mutually separate exhaust manifolds would be at least lessened.
A further disadvantage of the use of an activatable turbine of the type described above, in which activatable outlet openings are used as switching means, consists in that, upon the activation of the outlet openings for the purposes of activating the turbine, the exhaust-gas flow conducted through the second turbine abruptly decreases, as in each cylinder there is now a further outlet opening, specifically the activated outlet opening, available for the exhaust gas during the charge exchange. Upon the activation of the first turbine, the charge pressure generated by the second compressor then abruptly decreases. The torque drop associated with the drop in charge pressure is undesirable. In this respect, measures are desirable for improving the transient behavior of the exhaust-gas turbocharging arrangement upon the activation of the first turbine.
The inventors herein have recognized the above issues and provide an approach to at least partly address them. In one example, a method includes responsive to deactivation of a first turbine of a first turbocharger, deactivating an exhaust valve of a cylinder to flow exhaust gas from the cylinder to a second turbine of a second turbocharger, and adjusting a speed of the second turbocharger via an electric machine coupled to the second turbocharger in a generator mode. The method also includes, responsive to activation of the first turbine, activating the exhaust valve to flow exhaust gas from the cylinder to the first turbine and the second turbine, and adjusting the speed of the second turbocharger via the electric machine in an auxiliary drive mode.
In this way, the electric machine may be activated in either a generator mode or an auxiliary drive mode to absorb rotational energy of the second turbocharger or to supply rotational energy to the second turbocharger, when the first turbine is in the process of being deactivated or activated. By doing so, the exhaust-gas turbocharging arrangement and torque characteristic of the internal combustion engine, in particular the transient behavior, can be improved further.
In another example, boost fluctuations that may occur during activation or deactivation of the first turbine may be controlled by adjusting a wastegate of the second turbine and adjusting the cylinder charge amount via adjustment of valve timing, lift, and/or duration of the non-deactivated cylinder valves. An example method provided herein includes, responsive to a command to activate a deactivated first turbine, closing a wastegate coupled across a second turbine and adjusting a parameter of a first cylinder valve. The method further includes activating the first turbine by activating a second cylinder valve. In one example, the first cylinder valve may be a partially variable exhaust valve controlling a first opening of the cylinder fluidically coupled to the second turbine (and not the first turbine), and the second cylinder valve may be a deactivatable exhaust valve controlling a second opening of the cylinder fluidically coupled to the first turbine (and not the second turbine). In another example, the first cylinder valve may be an intake valve. In one example, adjusting the parameter of the first cylinder valve may include adjusting the timing, lift, and/or duration of a valve event of the first cylinder valve to reduce a volume of charge air admitted to the cylinder.
In this way, before the activation of the deactivated outlet openings, preparatory measures are performed in order to make it possible for a torque drop upon the activation of the first turbine to be counteracted in an effective manner, preferably without a delay.
Accordingly, the exhaust-gas flow rate conducted past the second turbine is reduced by virtue of the wastegate (also referred to as the second shut-off element) or the second bypass line being at least partially closed, wherein the additional power thereby generated at the second turbine, which is available as additional compressor power, increases, or would increase, the charge pressure at the inlet side. Specifically, an increased charge pressure is compensated according to the disclosure in that the cylinder fresh charge, that is to say the charge air mass remaining in the cylinders after the charge exchange, is set and metered, and in particular can be kept constant. The latter is performed using further at least partially variable valve drives, which are provided at the inlet side and/or at the outlet side.
An increased or higher charge pressure may be compensated for example by way of a reduction of the volumetric air usage. Specifically, the charge air mass supplied to a cylinder is dependent both on the charge pressure and on the supplied volume. That is to say, an increased or higher charge pressure can be compensated by reducing the volumetric efficiency λl, wherein the following applies:λl=mz/Mth=Vz·ρz/Vth·ρth=Vz/Vth The mass of the supplied cylinder fresh charge is denoted by mz and the theoretically suppliable cylinder fresh charge is denoted by mth, wherein, for the theoretical charge density ρth and the charge density ρz in the cylinder, the following approximately applies: ρth=ρz. The density ρ is determined in each case by the charge pressure. The theoretical air usage Vth is made up of the swept volume and the compression volume together.
If the demanded torque is to be kept unchanged, that is to say maintained, during the execution of the preparatory measures, it is necessary for the mass of the cylinder fresh charge to be maintained. In the presence of relatively high charge pressure, it is necessary for the at least partially variable valve drives to then be adjusted such that the volumetric air usage Vz, that is to say the volumetric cylinder fresh charge, to be reduced. Here, the second compressor is operated at a significantly higher rotational speed than would actually be required, and therefore has a present rotational speed reserve.
If the deactivated outlet openings are now activated for the purposes of activating the first turbine, the exhaust-gas flow rate discharged from the cylinders via the activated outlet openings, that is to say the exhaust-gas flow that is now absent at the second turbine, is compensated by setting the cylinder fresh charge by way of adjustment of the further at least partially variable valve drives, specifically to such an extent that the demanded torque is provided, or the presently prevailing torque is maintained.
The exhaust-gas flow conducted through the second turbine duly decreases when the first turbine is activated. According to the disclosure, however, the torque drop observed in the prior art as a result of a charge pressure loss is eliminated.
A torque drop upon the activation of the first turbine can be counteracted virtually without delay, because according to the disclosure, at the inlet side, that is to say at the side of the intake system, the cylinder fresh charge is influenced specifically using at least partially variable valve drives or by variation of the timing and/or of the valve lift of said valve drives.
This approach permits charge pressure control with very fast response. In relation to methods in which it is sought for the charge pressure to be increased or raised on the exhaust-gas side by adjustment of the second shut-off element, that is to say by closing the second bypass line, the method according to the disclosure for controlling the charge pressure has proven to be significantly faster.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.